Feedthrough with optic fiber sealed in protective tube and optical measurement apparatus using the same

ABSTRACT

The invention relates to an optical feedthrough including an optical fiber ( 11 ) and a protective tube ( 10 ) surrounding said fiber. In the invention, a seal ( 6 ) extends inside the annular space between the protective tube and the optical fiber, the length of said seal being greater than 50 mm.

The invention relates the field of optical feedthroughs. It relates moreparticularly to optical feedthroughs designed to interconnect twoelements situated in respective environments that are sealed relative toeach other.

Very often, apparatus containing optical fibers has the particularitythat, for the needs of taking measurements, the fibers are situated in awet or corrosive environment in which the pressure and the temperaturecan be high, while it is essential for the electronic equipment to whichthey are connected to be situated in a dry and clean environment andpreferably under pressure conditions that are close to atmosphericconditions. Generally, it is necessary not only to solve problems ofprotecting the fibers, in order to guarantee effective opticaltransmission, but above all to solve problems of sealing the couplingbetween the fibers and the electronic equipment which is often placed ina protective enclosure. Thus, it is necessary to ensure that strictseparation exists between the polluting environment surrounding thefibers and the environment inside the protective enclosure, while alsoallowing the fibers to penetrated into the protected environment.

Document U.S. Pat. No. 5,943,462 proposes an optical feedthrough thataims to solve the problem of sealing between the electronic equipment,situated in a protective housing, and the polluting environmentsurrounding the optical fiber. To that end, that document describes afeedthrough made up of two main portions. A first portion, through whicha channel passes that serves to pass the fiber into a protectivehousing, is screwed via one end to said housing while the other end isscrewed to the second portion, in the manner of a stopper. When the twoportions are screwed together, they flatten the elastomer seals servingto provide sealing. The fiber also passes through said second portion.

That solution is not really satisfactory. That feedthrough is firstlyparticularly complicated and requires a large number of parts: it istherefore difficult and costly to manufacture. Secondly, when the fiberis situated in surroundings that are particularly polluting and/or thatare under high pressure, the coupling provided by compressing theelastomer seals is not reliable. Such an optical feedthrough cannotwithstand being used at particularly high pressures such as thoseencountered in hydrocarbon wells.

Other coupling solutions that are simpler have been developed. Inparticular, direct coupling is known that is constituted by brazingbetween the fiber and the protective housing: the fiber passes throughthe wall of the housing via an orifice that is filled in with brazing.Those solutions are indeed very simple and inexpensive to perform, buttheir weakness lies in the difficulty of assembling together effectivelymaterials as different as the silica of the fiber and the metal of thebrazing. In addition, those solutions raise obvious maintenanceproblems. Since a brazed coupling cannot be disassembled, if any one ofthe elements of the feedthrough is damaged, the entire feedthrough mustbe replaced, which is impractical and above all costly.

An object of the invention is to remedy those drawbacks by providing anoptical feedthrough which guarantees excellent sealing between theelectronic equipment and the environment surrounding the fiber under allconditions, the feedthrough being particularly well-suited towithstanding high stresses, in particular due to the temperature, to thepressure, or to the aggressiveness of said environment.

To this end, the invention provides an optical feedthrough including anoptical fiber and a protective tube surrounding said fiber. In theinvention, a seal extends inside the annular space between theprotective tube and the optical fiber, the length of said seal beinggreater than 50 mm.

In this way, the feedthrough of the invention provides effectiveprotection for the fiber against a polluting outside environment, whilealso distributing the stresses exerted by this environment, inparticular those due to pressure. Since the seal extends inside theannular space between the tube and the fiber over quite a long distance,the chances of a leakage path being created by the seal beingdistributed poorly in the annular space are minimized, where such aleakage path would make it possible, in the event of the feedthroughbreaking, for polluting fluids to penetrate through said annular space.The optical feedthrough of the invention is thus particularlywell-suited to operating conditions under which the environmentsurrounding the electronic measurement instruments is very differentfrom the environment in which the measurements are taken.

In a preferred embodiment of the invention, the seal extends inside theannular space between the protective tube and the optical fiber, overthe entire length of said protective tube.

This embodiment makes it possible to guarantee total leaktightness forthe connections made by the optical feedthrough of the invention. It ishighly unlikely that polluting fluids can penetrate over such a lengthvia the annular space between the tube and the fiber to reach thesensitive means to which the fiber is connected. The longer the seal,the lower the chances of a path being created to said means by said sealbeing poorly distributed in the annular space. In addition, the presenceof the seal over the entire length of the fiber makes it possible toimpart greater strength to the feedthrough.

In an advantageous embodiment of the invention, the protective tube ismade of metal.

This embodiment makes it possible firstly to stiffen the opticalfeedthrough as a whole, and thus to increase its resistance to shocksduring handling. In addition, when the optical feedthroughs of theinvention are used in wells containing hydrocarbons or water, theprotective tube is particularly effective in these highly abrasive andcorrosive environments for avoiding any breaking of the optical fiber.Furthermore, since the protective housings surrounding the electronicmeans connected to the fiber are very often made of metal, it is easyand inexpensive to provide reliable coupling between the tube and thehousings if they are made of the same material.

In a preferred embodiment of the invention, the diameter of the fiber isclose to the inside diameter of the protective tube, so that thethickness of the seal is preferably less than 0.05 mm.

It is preferable for the thickness of the annular space to be limited soas to guarantee that the fiber is placed ideally inside the protectivetube. To reduce the probability of having a seal-free path through theannular space, it is necessary to ensure that the fiber occupies as muchspace as possible inside the tube, and thus for the seal also to bedistributed uniformly over the small available space, without anundesirable build-up in one place to the detriment of some other place,because of the space left unoccupied by the fiber.

In a preferred embodiment of the invention, the seal is a seal ofthermo-settable adhesive, said seal being polymerized in the protectivetube at a first temperature so that, at a second temperature that islower than said first temperature, said seal is compressed between thewalls of said tube and the optical fiber.

In this way, since the seal is polymerized inside the annular space atthe maximum temperature at which the feedthrough is to be used, then, atthe time it is polymerized, said seal occupies all of the availablespace in said annular space with the expansion of the protective tubebeing at its maximum. In which case, the seal is compressed against thewalls of the tube while the feedthrough is being used over an entirerange of temperatures extending to the maximum temperature (thepolymerization temperature). It is thus possible to limit the risks ofthe seal breaking, and particularly the risks of cracks forming in it,while maximizing the sealing of the annular space when the tube and thefiber are in the aggressive environment.

This embodiment also makes it possible to accommodate as well aspossible the differences in materials, and thus the differences inthermal expansion between the optical fiber and the protective tube. Theoptical fiber is made of silica whereas the tube is made of a completelydifferent material, e.g. a metal. In which case, when the environmentsurrounding the feedthrough is at high temperatures or at highpressures, the tube tends to expand to a greater extent than the extentto which the fiber expands. If the seal were not under compression, butrather at its maximum expansion, it would not be able to withstand suchdifferential expansion, and it would break. Whereas, in this embodiment,the seal expands naturally inside the annular space so that, withoutbreaking, it compensates for the differential expansion between the tubeand the fiber. The leaktightness of the annular space is thus guaranteedeven when the feedthrough is highly stressed, in particular due totemperature or pressure.

In another preferred embodiment of the invention, the seal extendsinside the annular space between the optical fiber and the protectivetube, at least from a first end of said fiber to a preferred zone ofweakness of said protective tube.

This solution makes it possible to control the place at which anybreaking takes place, and consequently to provide enough material toguarantee sealing when the fiber is connected to electronic measurementmeans. In which case, it is necessary merely for the seal to extend atleast as far as said preferred zone of weakness.

In an advantageous embodiment of the invention, the second end of thefiber is connected to an element situated outside a housing, saidfeedthrough including closure means co-operating with the protectivetube so as to seal off the inside of said housing relative to theoutside environment. In this feedthrough, the first end of the fiber issituated inside the housing, and the preferred zone of weakness issituated outside said housing.

The characteristics of the optical feedthrough of the invention make itpossible to provide effective sealing between an environment situatedinside a protective housing, containing electronic means for example,and the outside of the housing, where conditions are such that theywould damage said electronic means. By placing the preferred zone ofweakness outside the housing, and by ensuring that the seal extends fromthe inside of the housing to said zone of weakness, the chances ofelements penetrating into the housing in the event that the feedthroughbreaks are lowered considerably.

In an embodiment of the invention, the closure means include a sleevethrough which the protective tube passes.

This embodiment enables sealing relative to the outside environment tobe provided easily and reliably where the protective tube containing thefiber passes into the housing. The sleeve makes it possible to increasethe stiffness of the assembly in the vicinity of this critical zone. Inaddition, the use of the sleeve makes it possible to disassemble theoptical feedthrough of the invention. This makes it easy to change thefeedthrough in the event that it is damaged, and thus facilitatesmaintenance of the tool that it equips.

In a preferred embodiment of the invention, a first end of the sleeve issituated outside the housing and is coupled to the protective tube sothat the coupling corresponds to the preferred zone of weakness of saidtube.

This configuration makes it possible to control the place at which anybreaking of the optical feedthrough takes place, and consequently tocontrol the constraints for sealing the feedthrough. In addition, sincethe zone of weakness is offset relative to the sealing zone (the otherend of the sleeve), a “safety” length is retained that corresponds tothe length of the feedthrough, in order to preserve the leaktightness ofthe zone in which the protective tube and the fiber penetrate into thehousing.

In another advantageous embodiment of the invention, the sleeve is madeof metal, and it is connected by brazing to the protective tube which isalso made of metal.

In this way, the sleeve imparts additional stiffness to the coupling tothe housing. Since the housing is very often made of metal, it is easyto couple together the elements, the solution of brazing beingparticularly inexpensive.

Other advantages and characteristics of the invention appear moreclearly from the following description given with reference to the soleaccompanying drawing which is a longitudinal section view of anembodiment of an optical feedthrough of the invention.

The figure shows an optical feedthrough 1 serving to connect a firstelement 2 situated inside a housing 3 to a second element 4 situatedoutside said housing. In this embodiment, the element 2 is constitutedby electrical equipment for recording and analyzing data collected bythe element 4 which is an optical sensor, the optical feedthrough 1causing said data to flow between the sensor 4 and the equipment 2.

The optical feedthrough 1 is made up of a protective tube 10 surroundingan optical fiber 11, the resulting assembly passing from the outsideenvironment to the inside of the housing 3 via a hollow cylindricalsleeve 12. The tube 10 protects the fiber 11 from the outsideenvironment which may contain polluting and corrosive liquids. As shownin the figure, the tube 10 covers the fiber over its entire length fromthe inside of the housing to the element 4. The tube thus makes itpossible to guarantee maximum effectiveness for the fiber 11, whileimparting strength to the assembly.

A first end 14 of the sleeve 12 is made up of two portions 140 and 141.The first portion 140, situated outside the housing 3, abuts against thehousing via a collar 142 which acts as a positioning abutment onmounting the feedthrough on the housing. The portion 141 behind thecollar 142 is plugged into a sleeve-receiving channel 30 in the housing3. This portion is provided with annular grooves receiving O-ring seals5 that act to provide a sealing barrier relative to the outsideenvironment (more precisely, the second O-ring seal acts as a “backup”in case the first seal is damaged). The hollow sleeve 12 is providedwith a through bore opening out inside the housing 3. The assemblycomprising the protective tube 10 and the optical fiber 11 passesthrough this bore from the outside environment to the inside of thehousing, where it connects to the element 2.

The second end 15 of the sleeve 12 is rigidly coupled to the protectivetube 10. In one embodiment, both the sleeve 12 and the tube 10 are madeof metal, and the coupling is performed merely by brazing. This fixedand rigid coupling constitutes a preferred zone of weakness of thefeedthrough 1. When the feedthrough is placed in an aggressiveenvironment or when it is subjected to high pressures, it is thisweakened zone that bears the highest stresses and that thus runs thehighest risk of yielding. In this way, the place at which any breakingof the feedthrough occurs is controlled.

Making the sleeve and the tube of metal is advantageous not only becausethe assembly is then less costly, but also because it is then possibleto interconnect the two elements rigidly and effectively by simple meanssuch as brazing, as indicated above, or by any other rigid couplingmeans. It is also advantageous to provide the preferred zone of weaknesssome distance away from the place at which sealing is provided relativeto the housing 3. If the feedthrough breaks, the sleeve 12 and theremaining portions of the protective tube 10 and of the fiber 11 remainin place in the housing 3 and maintain the sealing relative to theoutside environment. Even if the feedthrough breaks, there is thus nodanger of the element 2 being damaged.

As shown in the figure, a seal 6 fills the annular space between theprotective tube 10 and the optical fiber 11 over the entire length ofthe assembly. In this way, the seal prevents any polluting fluids frompassing from the outside environment to the inside of the housing 3 viathe annular space between the fiber and the tube. In a preferredembodiment of the feedthrough of the invention, this seal 6 is a seal ofthermo-settable adhesive. This embodiment offers many advantages insofaras the coefficient of expansion and the Young's modulus of the opticalfiber, which is made of silica, are very different from those of theprotective tube, which is preferably made of metal. When the opticalfeedthrough is used in a high-temperature environment, the tube and thefiber expand to very different extents. Similarly, under high pressures,the tube is compressed differently to the optical fiber. It is thusnecessary to find a seal that can “absorb” the differential expansion orcompression without breaking. For this purpose, the seal 6 is thus aseal of adhesive that can be polymerized at the maximum temperature atwhich the optical feedthrough is used.

The seal 6 is then placed as follows.

The protective tube is filled with the chosen thermo-settable adhesive.The optical fiber 11 is inserted into the tube as filled with adhesive,the insertion being performed such that the risk of generating airbubbles in the tube and thus of generating places not filled withadhesive is as low as possible. The adhesive then becomes distributedthroughout the annular space between the fiber and the tube. Theassembly is then placed in an oven at the maximum temperature at whichthe optical feedthrough is to be used. Under the effect of the heat, thetube expands immediately while the fiber expands less, and the thicknessof the annular space is then at its maximum. The adhesive is thuspolymerized in this maximum space. Subsequently, when the assembly isremoved from the oven, the protective tube shrinks and compresses thepolymerized adhesive. In this way, when the feedthrough is used in arange of temperatures of up to the polymerization temperature, thestill-compressed seal of adhesive naturally tends to occupy the entireannular space, thereby absorbing the differential expansion between theoptical fiber and the tube. This technique is particularly advantageousbecause it makes it possible to use a seal between two very differentmaterials, subjected to variations in temperature and thus to variationsin geometrical shape, without ever weakening the sealing provided by theseal. In addition, at temperatures ranging up to the polymerizationtemperature, the seal is compressed against the walls of the tube. Byavoiding the risks of cracks forming or of leakage paths being created,this increases the strength of the seal over time.

It is important to note that the diameter of the fiber and the insidediameter of the protective tube are advantageously close to each other,so that the thickness of the seal of adhesive is minimized. Above all,the fiber must be positioned uniformly inside the tube if the spaceallotted to the fiber is very large, said fiber can stick against one ofthe walls of the tube, in which case the adhesive does not reach thatplace. Ideally, the fiber should be centered in the middle of the tubeso that the seal of adhesive is distributed uniformly inside the annularspace, and so that there is almost no place without adhesive. Thepurpose of this is to prevent “adhesive-free paths” form forming in thetube that could channel external fluids into the housing if thefeedthrough breaks. In order for the fiber to become substantiallycentered, the space available to it inside the tube must be minimized.Preferably, the thickness of the seal of adhesive is less than 0.05 mm.

As shown in the figure, if the feedthrough breaks in the zone ofweakness, the entire length of seal between the second end 15 of thefeedthrough and the inside of the housing prevents any ingress offluids.

It can thus be understood that it is the length of the seal in theannular space between the fiber and the protective tube that isessential: the longer this length, the lower the risk of a preferredpath through the adhesive leading to the inside of the housing. Even ifthe feedthrough breaks beyond the zone of weakness, sealing is stillguaranteed because the seal of adhesive extends over a sufficientlength. The length of the seal that is sufficient is 50 mm. Good resultshave been obtained for a length lying in the range 50 mm to 500 mm.Preferably said length is 300 mm. Beyond a certain length, althoughsealing is naturally guaranteed, problems can arise related to the lackof compactness of the feedthrough.

The preferred zone of weakness as shown in the figure is merely anembodiment of the invention. Provided that the length of the seal in theannular space between the fiber and the metal tube is sufficient toremove any risk of a preferred path running along the entire feedthroughto the inside of the housing or to the inside of the electronicequipment to which the feedthrough is connected, the optical feedthroughachieves the objectives of the invention.

In another embodiment, it is possible to make provision for the seal ofadhesive to extend to the second end 15 of the sleeve 12 only, i.e. tothe preferred zone of weakness. Furthermore, the sleeve 12 may bereplaced by a mere stopper. These solutions can be disassembled and arevery advantageous because, when the feedthrough is damaged, it is simpleand quick to separate the assembly from the housing in order to installa new optical feedthrough. Maintenance of the assembly is madeconsiderably easier.

In other embodiments, it is possible to use any seal other than a sealof thermo-settable adhesive, provided that said seal is capable of“absorbing” the differences in compression and/or in expansion betweenthe fiber and the protective tube, and provided that it is relativelyeasy to insert into the annular space between the fiber and the tube.For example, the seal may be in the form of a powder. It would thereforebe easy to distribute throughout the annular space, and the powder couldthen be polymerized by heating the assembly.

It is also possible to use any means other than a rigid coupling on theprotective tube to organize the preferred zone of weakness in some othermanner, such as a groove in the tube, a smaller-diameter portion, etc.

In other embodiments, a plurality of optical fibers are contained in theprotective tube, with a seal being distributed between them. For thesame reasons as above, it is preferable for the fibers to have as littlespace as possible in the tube so as to maximize the chances of the sealbeing distributed uniformly throughout the available space. In general,this is easy to achieve if the tube contains an odd number of opticalfibers. It is also possible to twist the fibers together before they areinserted into the protective tube.

The optical feedthrough of the invention has many possible uses in thefield of optical measurement, particularly when the measurement sensoris situated in an aggressive environment, at high temperatures and/orunder high pressures, and when the associated electronic equipment issituated in a protective housing. An example of a preferred use of theinvention relates to taking optical measurements down a well containinghydrocarbons, water, gas, or the like: in a hydrocarbon well, it ispossible, for example, to use the optical feedthrough of the inventionin apparatus for performing optical measurements to discriminate betweenthe three phases of a three-phase effluent on the basis of thedifference in the refractive indices of the phases. Downhole conditionsare so difficult (high temperature, high pressure, and corrosive andabrasive liquids, etc.) that considerable precautions must be taken toprotect the optical fiber and to connect the sensor to the electronicequipment protected inside a tool. The optical feedthrough of theinvention is then particularly well suited to these situations.

What is claimed is:
 1. An optical feedthrough including an optical fiberhaving a first end inside a housing, exposed to first conditions ofpressure and a second end connected to an element outside said housing,exposed to second conditions of pressure, said feedthrough furtherincluding a protective tube surrounding the fiber and defining anannular space between the protective tube and the optical fiber, wherebya seal having a length greater than 50 mm is provided inside saidannular space.
 2. The optical feedthrough of claim 1, wherein the sealextends inside the annular space between the protective tube and theoptical fiber, over the entire length of said protective tube.
 3. Theoptical feedthrough of claim 1, wherein the protective tube is made ofmetal.
 4. The optical feedthrough of claim 1, wherein the diameter ofthe fiber is close to the inside diameter of the protective tube, sothat the thickness of the seal is less than 0.05 mm.
 5. The opticalfeedthrough of claim 2, wherein the diameter of the fiber is close tothe inside diameter of the protective tube, so that the thickness of theseal is less than 0.05 mm.
 6. The optical feedthrough of claim 1,wherein the seal comprises a seal of adhesive, said seal beingpolymerized in the protective tube at a first temperature so that, at asecond temperature that is lower than said first temperature, said sealis compressed between the walls of said tube and the optical fiber. 7.The optical feedthrough of claim 2, wherein the seal comprises a seal ofadhesive, said seal being polymerized in the protective tube at a firsttemperature so that, at a second temperature that is lower then saidfirst temperature, said seal is compressed between the walls of saidtube and the optical fiber.
 8. The optical feedthrough of claim 1,wherein the seal at least extends from the first end of said fiber to apreferred zone of weakness of said protective tube.
 9. The opticalfeedthrough of claim 8, further including closure means co-operatingwith the protective tube so as to seal off the inside of said housingrelative to the outside environment, and the preferred zone of weaknessis situated outside said housing.
 10. The optical feedthrough of claim9, wherein the closure means include a sleeve through which theprotective tube passes.
 11. The optical feedthrough of claim 10, whereina first end of the sleeve is situated outside the housing and is coupledto the protective tube so that the coupling corresponds to the preferredzone of weakness of said tube.
 12. The optical feedthrough of claim 11,wherein the protective tube and the sleeve are made of metal, and thesleeve is connected by brazing to the protective tube.
 13. An opticalmeasurement apparatus to be used in a well containing hydrocarbons, gas,water or the like, which apparatus includes: electronic measurementmeans situated inside a protective housing exposed to a first pressure;an optical measurement sensor exposed to a second pressure; and anoptical feedthrough connecting said optical measurement sensor to saidelectronic measurement means; wherein said optical feedthrough comprisesan optical fiber and a protective tube surrounding said fiber, and aseal extending inside the annular apace between said protective tube andsaid optical fiber, the length of said seal being greater than 50 mm.